1. Field of the Invention
The present invention relates to safety bindings for skis, adapted to maintain, in a releasable fashion, one portion of a boot with respect to a ski.
2. Description of Background and Relevant Information
Safety bindings normally comprise at least one jaw adapted to retain a portion of a boot and to free it when subjected to excessive force exerted at the level of the leg of the skier. The jaw is movable, with respect to a fixed binding base, between a stable equilibrium position or rest position, and a released position in which the jaw frees the boot portion. In its movement from the stable equilibrium position towards the released position, the jaw, moved by the boot, is displaced against the force exerted by an energization assembly, the energization assembly tending then to assure the return of the jaw into the stable equilibrium or rest position. It is thus necessary to furnish a certain degree of energy to attain the release of the binding, i.e., its displacement to bring it into the release position.
In the most commonly used embodiments, the energization assembly is constituted by a prestressed spring. The release is obtained as soon as a predetermined force is reached. It is known that a spring acts in the same manner when it is subjected to a brief force or to a long force of the same amplitude. Thus, for such conventional bindings, the release threshold is the same whatever the duration of the force. The only factor which modifies the release threshold is the inertia of the mechanical elements which are made to move, which is most often negligible.
Yet, it is known that a leg can resist an elevated force of high amplitude if the duration of the force is brief. On the other hand, a force of low amplitude for an extended duration can be dangerous, for example in a slow fall. Thus, with a binding having only springs, the user is required to adjust the release threshold to a lower value or amplitude which is safe for a longer duration. However such bindings are not capable of withstanding high but acceptable forces of short duration, thus permitting early unnecessary or premature release.
It has appeared advantageous to design bindings in which the release threshold varies as a function of the duration of the force, the threshold being higher for brief forces and being lower for longer forces. It has been proposed, for example in French Pat. No. 1,454,511, to associate a fluid shock absorber with the principal spring of the energization means, positioned in parallel. In such an apparatus, when the force is slow, i.e., when the speed of displacement of the jaw is relatively slow, the shock absorber has a negligible frictional force relative to the return force exerted by the spring, and the release threshold is determined by the spring itself. On the other hand, when the force is brief, the speed of displacement of the jaw is substantial, and the resistance opposed by the shock absorber adds to the return force exerted by the spring, causing an increase in the energy necessary for release and in the force which must be applied on the jaws during release. Such a device has a major disadvantage, however, because the shock absorber has an infinite resistance when the speed of displacement of the jaw is very substantial in certain limited situations, i.e., when the force is a shock.
A similar solution was proposed in French Pat. No. 2,424,040, in which the spring is likewise in parallel with a shock absorber so that the extent of compression of the spring and that of the shock absorber are identical. In the event of a brief force being exerted, for example in the case of a brutal shock, the shock absorber produces a very elevated release threshold, which is in theory infinite.
It is likewise known to position a release spring and a shock absorber in series. Such a solution is proposed, for example, in German Pat. No. 2,634,649. In this case, if the shock absorber blocks up during a violent shock, the release threshold is raised, but the spring can still compress.
The disadvantage of this design is that it is necessary to equip the shock absorber with a return spring, which is at least as strong as the principal spring, in a manner so as to equilibrate the pressure of the principal spring in the rest position. In effect, it is necessary to prevent the principal spring from compressing the shock absorber at the end of its extent in the absence of the bias force. This poses a constructional problem which is all the more complex because it is necessary to be able to vary the prestress of the principal spring depending upon the performance of the skier.
Another disadvantage of this construction is that it is difficult in practice to determine the effect of the shock absorber on the principal spring, because the shock absorber is directly in series with the principal spring.